Class A Addresses
Class A addresses are used for very large networks with millions of hosts, such as the Internet. A Class A network number uses the first 8 bits of the IP address as its network ID. The remaining 24 bits make up the host part of the IP address. The value assigned to the first byte of a Class A network number falls within the range 0 to 127. For example, consider the IP address 75.4.10.4. The value 75 in the first byte indicates that the host is on a Class A network. The remaining bytes, 4.10.4, establish the host address. An RIR assigns only the first byte of a Class A number. Use of the remaining 3 bytes is left to the discretion of the owner of the network number. Only 126 Class A networks can exist because 0 is reserved for the network, and 127 is reserved for the loopback device, leaving 1 to 126 as usable addresses. Each Class A network can accommodate up to 16,777,214 hosts. The 10.x.x.x network is reserved for use by private networks for hosts that are not connected to the Internet. If you want to assign a Class A network and you are not visible on the Internet, you can use one of these network addresses.
Class B Addresses
Class B addresses are used for medium-size networks, such as universities and large businesses with many hosts. A Class B address uses 16 bits for the network number and 16 bits for the host number. The first byte of a Class B network number is in the range 128 to 191. In the number 129.144.50.56, the first 2 bytes, 129.144, are assigned by an RIR and make up the network address. The last 2 bytes, 50.56, make up the host address and are assigned at the discretion of the network's owner. A Class...